If it becomes a copy of a token creature, it copies the original characteristics of that token as defined by the effect that created the token. It won't become a token creature.

If the creature an opponent controls is an illegal target as the ability resolves, nothing happens and the creature you control remains as it was.

The target creature you control copies the printed values of the other creature plus any copy effects that have been applied to it. It won't copy any other effects that have changed that creature's power, toughness, color, and so on. It also won't copy any counters on the creature, but it will retain any counters it already had on it.

When a creature becomes a copy of another creature, it's neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won't trigger.